Many types of machinery are subject to vibration, which may cause the loosening of threaded fasteners in the machinery. In many applications, such as turbine engines, a loosened fastener can cause catastrophic damage to the machinery. It is therefore necessary to lock the threaded fasteners in the machinery in position so that they can not rotate in a direction which would tend to loosen and disengage them.
One locking system in the prior art utilizes two solid locking wires. Each of the fasteners to be locked has an aperture to receive a locking wire, and has one of the two wires passing through its aperture. The two wires are twisted together between fasteners and tensioned to prevent the fasteners from rotating in the loosening direction. The ends of the two wires are also twisted together and bent into a pigtail shape to prevent separation.
Instead of using two solid wires, a single solid wire may also be used to implement the twisted solid wire locking system. In that case the wire is first threaded through the aperture of the first fastener in the locking sequence. The wire is then doubled back around that fastener and the two halves of the wire are used in the same way as the two wires in the double wire system described above.
The twisted solid wire locking system has many drawbacks. For instance, because the wires must be twisted together during installation, it is very time consuming to install such systems. It is also difficult to maintain uniform tension in the wires. The quality of installation varies greatly due to the dependency on the skill of the operator.
An improved system in the prior art utilizes a single flexible multi-strand wire which is threaded through the aperture of each of the fasteners to be locked together. The wire is placed under tension as it is threaded through the fastener apertures, and the tension is maintained by affixing a crimped-on body on each end of the wire to prevent the ends from going through the terminal fasteners. Tools have been designed for the application of affixing the crimped-on body onto the locking wire. For example, U.S. Pat. No. 5,214,832 to Koehler et al. discloses a hand tool, and U.S. Pat. No. 5,127,144 to Plasse et al. discloses a hydraulic tool. The single flexible wire system has many advantages over the system with twisted solid wires, such as more uniform tension in the wire between fasteners, and much easier installation.
The single flexible wire locking system in the prior art has not been entirely satisfactory, however. In the prior art the crimped-on body on the end of the wire is deformable, but of a relatively thick construction so as to be large enough to interfere with the fastener aperture. To deform the crimped-on body substantially so as to firmly engage the locking wire requires substantial crimping force. In field applications the crimped-on body is often affixed to the locking wire with a hand tool. The high crimping force required increases the effort needed to install the locking system as well as the risk of acquiring carpal tunnel syndrome by workers performing many crimps a day. Power tools such as a crimper using a hydraulic punch are available, but they tend to be heavy and large, therefore less easy to handle than hand tools. The bulkiness of the power tools also makes it more difficult to reach into areas with limited access.
Another problem of the single wire locking system in the prior art is also related to the installation of the crimped-on body. .To assure that the body can be properly crimped onto the wire, the axial aperture in the body must be of a size which closely fits the locking wire. The close fit at relatively small dimension with a somewhat flexible wire may make it difficult to insert the wire into the body.